The present invention relates to intensity control of modulators used to form images on radiation sensitive media from a constant wave radiation beam. Although the invention can be applied to any system, it is particularly aimed at the imaging of printing plates sensitive to infrared radiation.
The use of lasers associated with optical modulators for the exposure of printing plates is well known. For example a laser beam is directed to an array of electrodes located on a substrate, allowing emerging sub-beams to selectively reach a radiation sensitive media such as a printing plate. The transmission of each beam to the image receptor surface is selectively inhibited in accordance with a predetermined pattern or program. In a machine developed by applicant the laser beam is divided into 256 individually controllable xe2x80x9cgatesxe2x80x9d allowing 256 pixels or light dots to reach the radiation sensitive surface or be prevented from doing so by stops or deflection means downstream of the modulator. However, the intensity of different pixels, may not be uniform along the line of pixels. Corrections should be introduced as found necessary following initial tests. Also, the intensity of various pixels constituting a light bar may vary during the operation of the imager. These variations may be caused by various factors for different reasons. Although they may be so small as to be acceptable for certain applications, they may not be acceptable for the production of high quality printing plates.
U.S. Pat. No. 4,780,731 describes a system for intensity control applied to a strip of light emitting diodes in which the regulation is obtained by affecting the duration of activated diodes. This system is not applicable to the production of bands or swaths as explained above because the exposure of a pixel is very short, of the order of 10 microseconds. The high frequencies necessary for such a mode are not possible in the present state of the art to control spatial electrooptic modulators.
The present invention relates to a method for the manual or automatic intensity control of each pixel or group of pixels by initial and periodic adjustment of the potential of gating electrodes. This adjustment is preferably accomplished by biasing the counter-electrodes individually or by groups without interfering with the control leads. This adjustment is independent of the control of selected marginal electrodes acting as an xe2x80x9celectronic maskxe2x80x9d as described in co-pending European Patent Application No. 99104942.0, which is incorporated herein by reference. The invention is not limited to a specific modulator. For example its scope covers PLZT as well as TIR modulators as either can accomplish the same function by similar means.
The present invention is applicable to the imaging of high quality thermal printing plates, for example, but without limitation, to machines and methods such as described in co-pending European Patent application No. 99112797.8, which is incorporated herein by reference.